Changes in an HER-2 peptide upregulating HLA-A2 expression affect both conformational epitopes and CTL recognition: implications for optimization of antigen presentation and tumor-specific CTL induction.

The HER-2/neu protooncogene (HER-2) is overexpressed in a significant number of breast and ovarian tumors. Peptides of HER-2 sequence were recently found to reconstitute recognition of cytotoxic T lymphocytes (CTLs) from tumor-associated (TALs) and tumor-infiltrating (TILs) lymphocytes, indicating that they reconstitute natural epitopes recognized by CTLs on HLA-A2+ tumors. Because HER-2 is an important antigen (Ag) for tumor-specific CTL induction and the immunogenicity of peptides for CTL induction is dependent on their presentation as stable complexes with HLA-A2, we identified peptides of high and low stabilizing activity from the sequence of HER-2 and the folate-binding protein (FBP). Distinct sequence patterns in the region positions (P)3-P5 and P1 were found for peptides with high (HSA) and low (LSA) stabilizing ability. A low-HLA-A2-affinity HER-2 peptide, P1 of the CTL epitope, was found to be permissive to substitutions that enhanced HLA-A2-stabilizing ability and conserved CTL recognition. In contrast, the region P3-P5 was not permissive to sequence changes. We conclude that the selective permissivity of P1 and P9 in the tumor epitope sequence may have important implications for optimization of tumor Ag presentation, and "neoantigenicity" of self-antigens, aiming toward induction of tumor-reactive CTLs of defined affinity and specificity for target Ags.